The invention relates to a display device comprising several electroluminescent semiconductor crystals situated on a base, a light conductor having apertures provided in a block being present above each of the electroluminescent semiconductor crystals, the upper side of the block being covered with a light filter.
From a point of view of manufacture the display devices may be divided into two large groups. In the first group the composing parts are incorporated in a resin which, after curing, ensures the mechanical coherence of the assembly; this resin fills all the intermediate spaces and in particular the light conductors. In the second group the component parts are united mechanically, for example, by soldering, and the light conductors generally contain only air.
The invention relates to this latter group of display devices of which an embodiment is disclosed in French Patent Specification No. 2,207,618 corresponding to U.S. Pat. No. 3,786,499. According to this patent specification the crystals are situated on a metal plate positioned on a substrate through which connection pins have been provided. Each crystal is connected to one of the said pins by means of a wire. An apertured block of plastic material is positioned above the plate and the crystals, each aperture being located opposite to a crystal and forming the light conductor of said crystal. The walls of the apertures are coated with a reflecting metal layer. The surface remote from the crystals of the apertured block is covered by a lid of a transparent plastic material which forms a light scattering lens. The side faces of this lid extends along the block and around the substrate so that the device is closed.
In the most recent devices of this type the metal plate is replaced by a printed circuit and the light conductive block is manufactured from a thermoplastic material having a pigment which has such a high reflecting power that the walls of the cavities need no longer be coated with a metal layer.
These known devices have the disadvantages of high production costs which is determined in particular by the manufacture of the printed circuit or substrate, the partial silver-plating or gold-plating of electric connection elements and the operations necessary for assembly.
It is also to be noted that the optimum conditions for the optical efficiency are not ideal due to on the one hand the partial screening by the presence of wire connections on the crystal and on the other hand the low reflectivity of the bottom of the light conductors, said bottom being sometimes coated with an anti-reflective layer, for example, in the device described in French Patent Specification No. 2,207,618, so as to attenuate disturbing optical interferences between adjacent light conductors.
An object of the instant invention is to provide a display device of the type described in the opening paragraph, the construction of which is simplified as compared to the prior art devices and in which is free of the above-mentioned technical disadvantages of the prior art devices. A further object of the invention is to provide a structure for such a display device by which it is possible to realize a considerable reduction in manufacturing cost. This and other aspects of the invention will be apparent from the description that follows.
According to the invention, a display device of the kind mentioned in the opening paragraph is characterized in that the base has laminated thereto electric connection elements manufactured from an electrically conductive thermo-setting resin, each of the electroluminescent semiconductor crystals bears on at least two of the electric connection elements on the base at the by input and output current regions provided laminated to a surface of said crystal, while the block bears on the base in contact with the laminated connection elements and that the base and the block are manufactured from the same thermoplastic material.
As a result of the fact that the base is manufactured from the same material as the block both parts have the same coefficients of shrinkage upon moulding and the same coefficients of expansion upon heating, which is favourable for the mutual cooperation of these parts after assembly.
The block and the base may be made from a thermoplastic material filled with a white-tinted pigment, which makes these parts opaque but gives the surface thereof, in particular the walls of the light conductors, reflective properties which are at least equal to those of the silver-plated or gold-plated surfaces. Since the block bears on the connection elements laminated to the base and as a result is so substantially in contact with the base, the advantage is obtained that the possibility of optical interference between adjacent light conductors is removed.
The use of the network of electric connection elements of a thermo-curing conductive resin results in a production cost of the device which is considerably lower than that of known devices in which printed circuit panels are used. Furthermore, a change of the network can more easily be carried out than in a printed wiring assembly panel.
Another aspect of the invention relates to the method of manufacturing a sub-assembly formed by the base, the network of connection elements and the semiconductor crystals connected to said network. According to this aspect of the invention in a first series of operations the network of electric connection elements is formed, for example, by silk screening, of an electrically conductive thermo-curable resin on a thermoplastic base, the electroluminescent semiconductor crystals are then placed on the still curable plastic resin with their current input and output surfaces suitably positioned on appropriate parts of the connection elements, and in a second series of operations the sub-assembly thus formed is then brought at such a temperature, in a given atmosphere and for a given period of time that the curing and connection of the electrically conductive thermo-curable resin to the base takes place simultaneously with the connection and the electric connection of the semiconductor crystals, after which the sub-assembly, the block and the light filter are combined.
The temperature and operation time may vary as a function of the nature of the resin used, on the understanding that the material of the base can withstand the treatment employed.
As a result the crystals are directly connected to the network of connection elements without the need for the use of wire connections which screen a part of the crystal.
The direct connection of the crystals therefore results in improving the brightness of the device by removing the cause of the partial screening.